1. Field of the Invention
The present invention relates to a polishing pad and a process for producing the same, and more particularly relates to an improved composite polishing pad has layers with different hardnesses and is so suitable for use in chemical-mechanical planarization (CMP) of semiconductor wafer, and a process for producing the polishing pad.
2. Description of Related Art
Chemical mechanical polishing (CMP) is typically used in a wafer planarization process to smooth the surface of a wafer so that the roughness and flatness of the wafer surface meet quality requirements. During the polishing process, the rough surface of a polishing pad, over which a polishing fluid is evenly distributed, is rotated on and makes frictional contact with the wafer surface, in order for the microparticles in the polishing fluid to grind the wafer surface and thereby produce the intended chemical reaction for flattening the wafer surface.
Both polishing pads and polishing fluids for use in CMP are consumables. When a polishing pad has been used for a while, its polishing (i.e., rough) surface is very likely to be uneven because it has been worn by wafer surfaces in frictional contact with it. If this worn polishing pad is still used in a CMP planarization process, not only the polishing speed but also the polishing result will be compromised, leaving the polished wafer surfaces with undesirable flatness.
A polishing pad or polishing surface made of a fibrous fabric impregnated with a polyurethane (PU) resin has relatively low hardness, relatively low porosity, and consequently a poor planarization effect. Lacking a substrate with a high-hardness and high-porosity surface and a highly compressible bottom layer, such a pad structure does not possess the physical properties generally required for CMP. More specifically, the polishing surface of such a pad structure tends to be uneven due to non-uniform distribution of the ingredients of the substrate, thus impairing the CMP effect and efficiency. The use of such pad structures is therefore limited.
In view of the above, one technical means to increasing the flatness of a wafer surface is to use a polishing pad with an abrasion-resistant high-porosity rough surface and a highly compressible (or buffering) bottom layer for maintaining the flatness of the rough surface after long-term use without any adverse effects on the polishing speed of CMP operation.
Nowadays, polishing pads in common use can be generally categorized as one-layer or composite. One-layer polishing pads are disadvantaged by their having only the rough surface, impregnated with a PU resin for example. Even though more advanced versions with an abrasion-resistant high-porosity rough surface have been developed, e.g., with a rough surface impregnated with a PU foam, the pads themselves are still not highly compressible and hence fail to provide the desired buffer function. Moreover, it is difficult to control the pore size of the foam and much more difficult to control the distribution of pores. When used in a CMP operation, this kind of polishing pads will have problem keeping the flatness of their rough surfaces; as a result, the flatness of the polished wafer surfaces falls short of quality requirements.
Composite polishing pads, on the other hand, have a stacked structure composed of at least two different polishing substrates. For example, the upper layer of a two-layer polishing pad is a relatively hard polishing substrate, has an abrasion-resistant high-porosity rough surface, and serves as an incompressible polishing layer while the lower layer is a relatively soft polishing substrate, features high compressibility, and functions as a buffer layer for keeping the flatness of the rough surface of the upper polishing layer.
In U.S. Pat. No. 5,287,663, Pierce et al. disclose a CMP pad formed by bonding together three layers of different materials. A top layer is a relatively incompressible polishing layer attached to a middle rigid layer. The middle rigid layer is formed of an incompressible material in order to provide rigidity and is disposed on a bottom resilient layer. The resilient layer is made of a compressible material in order to impart resilience to the rigid layer.
The drawbacks of such composite polishing pads stem from the fact that their multilayer pad structure is not integrally formed but is constructed by bonding the working layers with intervening adhesive layers. During a CMP operation, the adhesive layers between the working layers are subject to shear forces caused by friction and may therefore be torn or peeled off, and the polishing layer of a polishing pad with dislodged adhesive layers cannot maintain a flat rough surface, let alone produce wafer surfaces of the required flatness.
Another type of composite polishing pads includes a polishing layer embedded with a buffer layer made of a compound with rubber elasticity, wherein the buffer layer has a smaller storage modulus than the polishing layer. As the buffer layer is formed inside the polishing layer, the properties of the polishing layer must be taken into account, which imposes limitations on the selection the buffer layer. Furthermore, the manufacture of such a polishing pad involves a complicated process.